MEASURING REFRACTION INDICES— POLARIZING MICROSCOPES 203 



to the left (i7i), rises. In the area BB', the path difference decreases. 

 If the initial tint (J being = 0) is still lavender grey, it now tends 

 to a darker tone. 



Zl=0 ! 



A--0 



I, 



I I 



FlG. 7.32. The two wave surfaces when // > n . 



In this way, the path difference at Bh.^ (a in equation 7.12) and 

 at BB' (b in equation 7.12) are measured unquestionably and it is 

 also ascertained that the surface wave (Z^ or Zo) is either the one 

 shown in Fig. 7.30 if /;' > n or the one in Fig. 7.31 if u < u. 



6. FRINGE-SHIFT PATH-DIFFERENCE MEASUREMENTS 

 (DIFFERENTIAL PROCESS) 



A system of straight, parallel and equidistant fringes can be made 

 visible in exactly the same way as with full-duplication microscopes 

 (§ 2). These fringes are always the loci of equal path differences 

 between the waves H^ and Z.^. As in full-duplication microscopes, 



Fig. 7.33. Fringe-shift path-difference measurements in the differential process. 



the fringes are paralleled to duplication. The object is so set that 

 the projection of the slope BB' on the horizontal plane is paralleled 

 with the fringes. The object in Fig. 7.25 has the aspect shown in 

 Fig. 7.33. The two bands BB' and CC are no longer correlated with 



